1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as circuit breakers. The invention also relates to heater assemblies for electrical switching apparatus.
2. Background Information
Electrical switching apparatus, such as molded case circuit breakers, generally include at least one pair of separable contacts which are operated either manually, by way of a handle disposed on the outside of the circuit breaker housing, or automatically by way of a trip unit in response to a trip condition (e.g., without limitation, an overcurrent condition; a relatively high level short circuit or fault condition; a ground fault or arc fault condition).
Relatively small molded case circuit breakers, for example, that are used in residential and light industrial applications, typically include a thermal-magnetic trip unit having a heater assembly and a magnetic trip mechanism. As shown in FIG. 1, the heater assembly 1 includes a plurality of heater elements 3,5 and a bimetal 7. In operation, for example in response to an overload condition, electric current drawn by the load (not shown) heats the heater elements 3,5 which, in turn, heat the bimetal 7 causing it to bend (e.g., in the direction of arrow 9 of FIG. 1) in a well known manner. When the bimetal 7 bends, it cooperates, directly or indirectly, with a trip bar 11 (shown in simplified form in phantom line drawing in FIG. 1) of the circuit breaker operating mechanism 13 (shown in block form), thereby moving (e.g., pivoting) the trip bar 11, which unlatches the operating mechanism 13 to open (e.g., separate) the separable contacts 15,17 of the circuit breaker (not shown) and interrupt the flow of electric current. Thus, the heater assembly 1 functions to provide a thermal trip response that is directly related to the magnitude of current drawn by the load.
Among other disadvantages, the structure and, in particular, the length 19 of the bimetal 7 is constrained by the limited amount of space that is available within the circuit breaker housing (not shown). The limited length 19 of the bimetal 7 constrains the mechanical advantage provided by the bimetal 7, when it bends. Accordingly, the amount of force exerted by the bimetal 7 on the trip bar 11, when the bimetal 7 is heated and bends, is undesirably limited. Additionally, the heater elements 3,5 must be suitably coupled (e.g., welded) together, which increases the complexity and cost of the heater assembly 1. The connection (e.g., weld) between the heater elements 3,5 can also provide thermal resistance, which adversely affects the thermal performance of the circuit breaker (not shown).
There is, therefore, room for improvement in electrical switching apparatus and in heater assemblies therefor.